Manufacture of aldol



Patented Aug. 1, 1950 MANUFACTURE or ALDoL Samuel Aaron Miller and Alan Roger Hammond, Spondon, near Derby, England, assignors to Celanese Corporation of America, a corporation of Delaware No Drawing. Application February 1, 1946, Se-

rial No. 645,003. In Great Britain February 16,

This invention relates to the aldol from acetaldehyde.

1 It has long been known that undersuitable conditions acetaldehyde is converted into aldol according to the equation: l

A considerable number of processes for carrying out this reaction have been put forward at different times. It is known for instance that both acids and alkalies are capable of catalysing the reaction, although most of these processes make use of alkaline catalysts such as the hydroxides and carbonates of the alkali metals and alkaline earth metals. Furthermore these older processes between them cover a wide range of temperatures and other operating conditions, and many different expedients have been proposed for ensuring a good yield of aldol and reducing to a minimum the formation of polymeric and other by-products.

Although in a number of these known processes the yield. of aldol (i. e. the proportion of the acetaldehyde consumed which is converted into aldol) is claimed to be fairly high, e. g. of the order of 80%, and the amount of by-products formed correspondingly low, the conversion of acetaldehyde to aldol (i. e. the proportion of the acetaldehyde submitted to the reaction conditions which is converted into aldol) is always comparatively low, a considerable amount of acetaldehyde being recovered unchanged. Indeed it has for, a long time been believed that some mechanism operated tolimit the proportion of the acetaldehyde which manufacture of can be converted to aldol to a theoretical maxi mum of 66 /370; in practice the conversions were always considerably lower owing to the occurrence to some extent of side reactions. Thus McLeod [Am. Ch. J. 37, 2741907)] stated that.

the highest yields which he could obtainwere 48-50%. (He does not state Whether or not, in calculating the yield, he took into account the fact that some acetaldehyde remained unreactedi. e. whether the figures given represent the true yield or the conversion. If the true yield.

is meant, the conversion must have been considerably lower than 48-50%.) McLeod used as catalyst potassium carbonate; for the first half hour after the addition of the catalyst to the acetaldehyde the temperature was kept below C. and after this the mixture was allowed to stand at room temperature for 18 hours. He stated'that further standing, for example for a total of 48 hours, actually decreased the amount of aldol obtained by more than a third, and increased the amount of oily by-products.

7 claims. (or. 260--602) Later work appeared to confirm the existence of this upper limit to the proportion of acetaldehyde which can be converted to aldol. Such a limit could be the result either of an equilibrium reaction or of the formation of some sort of loose compound between molecular proportions of aldol and acetaldehyde, but the latter alternative was considered the more probable. lHibbert, J. A, C. S;, 37, 1748 (1915); Kagan, Lyubarski, and Fedorov, J. App. Ch. U. S. S. R. '7, (1934).]

We have now found that, in spite of the statements of the workers mentioned above, it is possible to achieve conversion figures much above those previously attained, and indeed even above the figure 06% previously believed to be the maxhigher temperature; the second consists in allowing the products of the first stage to stand for a comparatively long period at about room temperature. It ismost remarkable, in view of the general knowledge of the art as outlined above, that during this second stage the conversion figure gradually rises so that a far greater proportion of the original aldehyde is recovered in the form of aldol than has up to now been believed possible, and that this increase in conversion can be achieved without any serious loss of yield by-the formation of oily or resinous by-produets, for example as described by McLeod.

According to the invention acetaldehyde which is free from acid or contains less than 0.1% of acid (calculated as acetic acid) is intimately mixed with sodium hydroxide in amount such that, after any acid has been neutralised, the amount of sodium hydroxide in the mixture is 0.15 to 0.25% of the weight of the acetaldehyde, or with a chemically equivalent amount of 811- other alkali metal hydroxide, and the mixture in the absence of free oxygen is kept first at a temperature of 40-50 C. for a period less than six hours but long enough to give a product in which the ratio of the amounts of aldol and acetalde-= hyde is higher than 1:2, and then at a tempera ture of 5-30 C. for at least 15 hours.

It is an important feature of the new process that the acetaldehyde to be converted into aldol should be as free from acid as is practicable, and i as stated above it is essential that its acid content (calculated as acetic acid) should not exceed 0. 1%. The acids usually found in acetalclehydeare acetic acid and peracetic acid. Even though these and any other acids present are completely neutralised by the alkali metal hydroxide, the neutralised acids still exert an unfavourable elfect both on the yield of aldol and On the conversion figure.

The proportion of alkali metal hydroxide in the mixture is also important; if the proportion is below the specified range the high conversion figures characteristic of the new process cannot be attained; if it is above this range not only the conversion figure but also the yield of aldol suffers. We prefer to employ sodium hydroxide in amount 0.18 to 0.22% of the weight of the acetaldehyde (in addition to such amount as may be neutralised by acid in the acetaldehyde), or a chemically equivalent amount of potassium hydroxide. In order to facilitate rapid and intimate mixing of the acetaldehyde and alkali metal hydroxide, the latter is best employed in a dilute aqueous solution, e. g. a solution of concentration about 2 to 6%, and especially about 4%.

In the first stage of the reaction the mixture is preferably kept at a temperature of 42?-47 C. and especially at about 45 (3.; it may be kept at this temperature for 1-3 hours, and should be Well stirred all the time. In the second stage the products of the first stage may be allowed to stand at room temperature, e. g. at 10-25 (3., for 18 to 24 hours with good results, as shown by conversion figures exceeding 66 75. By prolonging this stage, for example to 3-10 or days, the conversion figure may be raised even higher. Both stages may very conveniently be carried out in an atmosphere of nitrogen.

In a preferred method of putting the invention into practice the first stage is carried out in a continuous manner, the second being continuous or discontinuous as is the more convenient. For example the acetaldehyde and the catalyst solution may be separately fed, continuously or intermittently, into a reaction vessel which is kept nearly filled with the reaction mixture and in which the mixture is continuously stirred, while the products of the reaction are withdrawn at substantially the same rate. Preferably the acetaldehyde and the catalyst solution are introduced, and the products are withdrawn, at or near the bottom of the vessel. The temperature is kept at the required level by any suitable cooling means. Advantageously the vessel is provided with a coil or jacket through which a cooling fluid (or at the inception of the reaction a heating fluid) can be passed, but other cooling means may be employed as well as or instead of a cooling coil or jacket. For example the acetaldehyde may be allowed to boil and be recondensed in a reflux condenser kept at the reaction pressure. The products after leaving the reaction vessel may be fed to a vessel or series of Vessels in which they are allowed to stand for the desired time, or they may be passed slowly through a suitable tube or column, or series of tubes or columns.

On the other hand the invention may be put into practice as a batch process throughout both stages. For example, the acetaldehyde may first be introduced into an autoclave provided with cooling means and a stirrer, and the catalyst solution slowly run in while the temperature of the contents of the autoclave is kept at the desired level. When the predetermined amount of catalyst solution has been added and the evolution of heat has become relatively slow, the contents of the autoclave may be allowed to cool and may either .be allowed to standfor the desired period in the autoclave or may first be transferred to another vessel.

At the end of the second stage of the reaction the catalyst is neutralised by the addition of a suitable acid. We have found that the best results are obtained when the catalyst is neutralised by the addition of phosphoric acid, for example in -70% aqueous solution, in amount equivalent to the amount of alkali metal hydrox-' ide which was mixed with the acetaldehyde, including any neutralised by acid impurities in the acetaldehyde. Other acids can however be employed, for example acetic acid in amount equivalent to the amount of alkali metal hydroxide added, excluding any neutralised by acid in the acetaldehyde. After the catalyst has been neutralised the reaction mixture may be freed from unreacted acetaldehyde by distillation under a pressure of about mm. or less; preferably this distillation ofv the acetaldehyde is carried out as far as possible at about room temperature, but generally it will be necessary finally to raise the temperature somewhat if it is desired to remove substantially all the acetaldehyde. In any case it is advisable not to heat the reaction mixture to a temperature above 50 C.

The crude aldol remaining after the removal of the acetaldehyde may for some purposes (e. g. dehydration to crotonaldehyde or reduction to 1:3-butane-diol') be employed without further purification, but if desired a pure product may be obtained by fractionally distilling the crude aldol under low pressures, for example pressures of about 5 to 15 mm.

It is not necessary that the acetaldehyde em-.

ployed as a starting material should be pure; it may for example contain water, and it may be in admixture with pure aldol or with crude aldol obtained from a previous run. It is quite satisfactory to use acetaldehyde of a usual industrial An autoclave provided with a stirrer and a coil for the passage of a heating or cooling fluid is evacuated and then filled with nitrogen. Acetaldehyde containing 0.04% of acetic acid and less than 0.001% of peracetic acid, and a 4% aqueous solution of sodium hydroxide as catalyst, are separately introduced near the bottom of the autoclave in the ratio of 15.5 parts by weight of acetaldehyde to 1 part of the catalyst solution. To initiate the reaction warm water is passed through the coil, but after the reaction has begun this is replaced by cold water, by means of which the temperature of the liquid in the autoclave is kept at 45 C. As soon as enough of the acetaldehyde and catalyst solution have been introduced nearly to fill the autoclave, liquid is withdrawn from the bottom of the autoclave at approximately the same rate as it is introduced, 50 keeping the volume 01 liquid in the autoclave more or less constant. Throughout the process the liquid in the autoclave is vigorously stirred. The rate of feed and thus also of discharge are such that the reactants remain in the autoclave for an average'period of two hours.

The liquid withdrawn from the autoclave is collected under nitrogen in a series of Vessels in which it is allowed to stand for seven days at about 15 :to 20 70 Enough of a 50% aqueous solution of phosphoric acid to neutralise the catalyst is then mixed into the liquid, and unchanged acetaldehyde is removed by distillation under 50 mm. pressure to a final liquid temperature of 50 C. The residue consists of aldol of a very satisfactory degree of purity in amount corresponding to about 79% of the acetaldehyde fed to the reaction. Moreover 16% of the original acetaldehyde is recovered unchanged.

Analysis of the reaction liquid after it has stood for only twenty hours shows that by that time about 70% of the original acetaldehyde has been converted into aldol, while 27% remains unchanged.

Having described our invention, what we desire to secure by Letters Patent is:

1. Process for the manufacture of acetaldol, which comprises intimately mixing acetaldehyde containing less than 0.1% of acid (calculated as acetic acid) with an alkali metal hydroxide in amount such that after any acid has been neutralized the amount of alkali metal hydroxide in the mixture (calculated as sodium hydroxide) is 0.15 to 0.25% of the weight of the acetaldehyde, keep-- ing the mixture in the absence of free oxygen at a temperature of 40 to 50 C. for a period less than 6 hours but long enough to give a product in which the ratio of the amount of acetaldol to that of acetaldehyde is higher than 1:2, and then at a temperature of 5 to 30 C. for at least hours.

2. Process for the manufacture of acetaldol, which comprises intimately mixing acetaldehyde containing less than 0.1% of acid (calculated as acetic acid) with sodium hydroxide in amount such that after any acid has been neutralized the amount of sodium hydroxide in the mixture .3

is 0.15 to 0.25% of the weight of the acetaldehyde, keeping the mixture in the absence of free oxygen at a temperature of 42 to 47 C. for a period less than 6 hours but long enough to give a product in which the ratio of the amount of acetaldol to that of acetaldehyde is higher than 1:2, and then at a temperature of 18 to C. for more than 18 hours.

3. Process for the manufacture of acetaldol, which comprises intimately mixing acetaldehyde containing less than 0.1% of acid (calculated as acetic acid) with sodium hydroxide in amount such that after any acid has been neutralized the amount of sodium hydroxide in the mixture is 0.15 to 0.25 of the weight of the acetaldehyde, keepin the mixture in the absence of free oxygen at a temperature of 42 to 47 C. for a period less than 6 hours but long enough to give a product in which the ratio of the amount of acetaldol to that of acetaldehyde is higher than 1 :2, and then at a temperature of 18 to 25 C. for 3 to 10 days.

4. Process for the manufacture of acetaldol, which comprises intimately mixing acetaldehyde containing less than 0.1% of acid (calculated as acetic acid) with an alkali metal hydroxide in amount such that after any acid has been neutralized the amount of alkali metal hydroxide in the mixture (calculated as sodium hydroxide) is 0.18 to 0.22% of the weight of the acetaldehyde, keeping the mixture in the absence of free oxygen at a temperature of to 50 C. for a period less than 6 hours but long enough to give a product in which the ratio of the amount of acetaldol to that of acetaldehyde is higher than 1:2, and then at a temperature of 5 to 30 0.101 at least 15 hours.

5. Process for the manufacture of acetaldol, which comprises intimately mixing acetaldehyde containing less than 0.1% of acid (calculated as acetic acid) with an aqueous solution'of sodium hydroxide of concentration 2.5 to 6% in amount such that after any acid has been neutralized the amount of sodium hydroxide in the mixture is 0.15 to 0.25% of the weight of the acetaldehyde, keeping the mixture in the absence of free oxygen at a temperature of 40 to 50 C. for a period less than 6 hours but long enough to give a product in which the ratio of the amount of acetaldol to that of acetaldehyde is higher than 1:2, and then at a temperature of 5 to 30 C. for at least 15 hours.

6. Process for the manufacture of acetaldol, which comprises feeding acetaldehyde containing less than 0.1% of acid (calculated as acetic acid) and an aqueous solution of sodium hydroxide of concentration 2.5 to 6% separately into a reaction vessel in proportions such that the resulting mixture after any acid has been neutralized contains 0.15 to 0.25% of sodium hydroxide (calculated on the weight of the acetaldehyde) stirring the mixture in the reaction vessel and maintaining it at a temperature of 40 to 50 0., continuously withdrawing liquid from the bottom of the reaction vessel, the rates of feed and withdrawal being approximately equal and such that the reactants remain in the reaction vessel for an average period of 1 to 3 hours, and allowing the liquid withdrawn to stand at room temperature for at least 15 hours.

7. Process for the manufacture of acetaldol, which comprises feeding acetaldehyde containing less than 0.1% of acid (calculated as acetic acid) and an aqueous solution of sodium hydroxide of concentration 2.5 to 6% separately into a reaction vessel in proportions such that the resulting mixture after any acid has been neutralized contains 0.18 to 0.22% of sodium hydroxide (calculated on the weight of the acetaldehyde) stirring the mixture in the reaction vessel and maintaining it at a temperature of 42 to 47 C., continuously withdrawing liquid from the bottom of the reaction vessel, the rates of feed and withdrawal being approximately equal and such that the reactants remain in the reaction vessel for an average period of lto 3 hours, and allowing the liquid withdrawn to stand at room temperature for more than 18 hours.

SAIVIUEL AARON MILLER. ALAN ROGER HAMMOND.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,437,139 Grunstein Nov. 28, 1922 1,598,522 Herrly Aug. 31, 1926 2,428,846 Hull Oct. 14, 1947 OTHER, REFERENCES Ser. No. 375,015, Szlatinay (A. P. (3.), published April 20, 1943.

Ser. No. 375,016, Szlatinay (A. P. C.) published April 20, 1943. 

1. PROCESS FOR THE MANUFACTURE OF ACETALDOL, WHICH COMPRISES INTIMATELY MIXING ACETALDEHYE CONTAINING LESS THAN 0.1% OF ACID (CALCULATED AS ACETIC ACID) WITH AN ALKALI METAL HYDROXIDE IN AMOUNT SUCH THAT AFTER ANY ACID HAS BEEN NEUTRALIZED THE AMOUNT OF ALKALI METAL HYDROXIDE IN THE MIXTURE (CALCULATED AS SODIUM HYDROXIDE) IS 0.15 TO 0.25% OF THE WEIGHT OF THE ACETALDEHYDE, KEEPING THE MIXTURE IN THE ABSENCE OF FREE OXYGEN AT A TEMPERATURE OF 40 TO 50*C. FOR A PERIOD LESS THAN 6 HOURS BUT LONG ENOUGH TO GIVE A PRODUCT IN WHICH THE RATIO OF THE AMOUNT OF ACETALDOL TO THAT OF ACETALDEHYDE IS HIGHER THAN 1:2, AND THEN AT A TEMPERATURE OF 5 TO 30*C. FOR AT LEAST 15 HOURS. 